1. Field of the Invention
The invention is concerned with devices for the storage of energy, especially of energy available in the form of heat.
2. Description of the Prior Art
The need for energy storage arises in a variety of contexts and on a variety of scales. For example, the problem arises on a small scale with artificial satellites depending on solar energy. Unless equipped with an energy storage device the satellite is without power whenever it is in darkness. On a larger scale, the problem arises with conventional or nuclear power plants. Since such plants are most economically operated at a constant power level, an energy storage device is needed to store energy when demand is low and to release energy when demand is high. One approach in current use is based on converting the energy to be stored into gravitational energy by pumping water into an elevated reservoir when excess energy is available and to use the water to drive a hydroelectric power plant when energy is needed.
An example of such an installation is the storage facility at Ludington, Michigan whose pumps and generators are connected to Michigan's network of power lines. However, due to the need for large tracts of land of suitable topographic situation, this method for storing energy cannot be expected to find wide application.
Another approach currently being studied is based on storing heat directly in the form of latent heat of fusion of a material at its melting point. For this purpose R. J. Hanold et al, in U.S. Pat. No. 3,029,596, advocate the use of lithium salts such as lithium hydride, lithium hydroxide and lithium fluoride. Energy to be stored in the heat reservoir causes some of the solid phase material to melt and energy withdrawn causes some of the liquid phase material to solidify. The selection of any specific material has to be based on its melting point, which ideally should be slightly below the temperature at which heat to be stored is available. Furthermore, for the sake of compactness of the device, the specific heat of fusion of the material should be high. Furthermore, since a high heat of fusion typically goes together with a high melting point, elevated operating temperatures are desirable.
Heat storage devices based on salts such as the lithium compounds mentioned above are much less space consuming than water reservoirs; however, certain difficulties arise with the placement of the heat exchanger out of the fact that the liquid phase of such materials is lighter than the solid phase. It is desirable to effect the exchange of heat between the heat storage medium and the heat source on the one hand, and the heat storage medium and the heat sink on the other, by a compact heat exchanging device in contact with a horizontal layer of the heat storage medium. However, when a salt eutectic is used as heat storage medium, the placement of the heat exchanger poses a dilemma. Specifically, placing the heat exchanger at the bottom of the reservoir, that is, at least partly inside the solid phase of the storage material, leads to a low rate of heat transfer and to mechanical stress during change of phase. Conversely, placing the heat exchanger at the top of the reservoir causes the insertion of heat to be impaired by lack of convection because the heat exchanger is in contact with the warmest layer of the heat storage medium. Even if the heat exchanger were raised and lowered in the liquid phase material depending on whether heat was to be withdrawn or inserted, convection in the liquid phase would counteract rather than promote the exchange of heat between the liquid phase and the solid phase.